The present invention relates to a rubber-erasable aqueous ink for a writing material composition and writing materials including same incorporated therein. More particularly, the present invention relates to a rubber-erasable aqueous ink for a writing material composition which gives onto paper writing that can be easily erased by an eraser but can be retained thereon even when normally rubbed with finger or paper. Even more particularly, the present invention relates to a rubber-erasable aqueous ink for a writing material composition which, when heated, exhibits a permanent fixability such that writing cannot be erased even when vigorously rubbed.
Many proposals have been heretofore made on ink compositions which provide writing that can be erased by an eraser. These ink compositions can be roughly divided into two groups, i.e., oil ink including a colorant and a vehicle containing a resin dissolved in an organic solvent as a main solvent and aqueous ink including these components dissolved in water as a main solvent. Examples of the oil ink include rubber-erasable aqueous ink for a writing material compositions for ball point pen including a pigment, a specific organic solvent and an elastomer resin having rubber elasticity as disclosed in U.S. Pat. Nos. 4,389,499, 4,390,646 and 4,391,927. The foregoing ink composition includes a resin having a high molecular weight and thus exhibits a high viscosity. In this arrangement, it is tried to prevent the pigment from penetrating deep into paper together with the solvent, making the writing formed by the pigment on paper erasable by an eraser. However, the foregoing ink composition can be practically erased by an eraser for a short period of time after writing but can be erased less gradually with time.
Another disadvantage of the foregoing oil ink composition is that since the solvent used is hardly volatile and thus is normally slow-drying after applied to paper, the ink composition which has just been applied, even when lightly rubbed with finger or hand, causes the undried portion to move to blank area (area around the writing), causing remarkable stain on paper as well as on finger or hand, even on clothing On the other hand, many proposals have been recently made on rubber-erasable aqueous ink for a writing material composition including water as a main solvent. One of these proposals is a rubber-erasable ink composition including a pigment in an amount of from 1 to 50%, a resin having a film-forming temperature of not lower than 40xc2x0 C. in an amount of from 3 to 50%, an organic solvent in an amount of 0.5 to 50% and water in an amount of from 7 to 60% based on the total weight of ink as disclosed in JP-A-4-332776. Since the foregoing ink composition includes a general-purpose pigment that can easily penetrate deep into paper, it can be fairly erased by an eraser. Further, since the resin cannot form a film at room temperature, the film (writing) can be poorly preserved.
Another proposal is an erasable ink composition including a water-insoluble polymer-capsuled colorant obtained by subjecting at least one emulsion-polymerizable monomer including a colorant dissolved therein to polymerization under emulsion polymerization conditions as disclosed in U.S. Pat. No. 5,661,197. In this proposal, a capsuled colorant including a colorant incorporated in a water-insoluble polymer is prepared under emulsion polymerization conditions. The resulting capsuled colorant includes finely divided particles having a diameter of not more than 1 xcexcm. As a result, when the ink composition is applied to paper, the finely divided colorant particles penetrate deep into paper and are trapped by paper. Thus, the foregoing ink composition can be poorly erased by an eraser. Further, the water-insoluble dye to be incorporated in the capsuled colorant is liable to extraction in the aqueous vehicle with the water-soluble polar solvent or surfactant in the aqueous vehicle. The dye thus extracted dyes the surface of paper to an extent such that the trace of writing can be left unerased even when rubbed with an eraser.
A further proposal is an erasable ink having a viscosity of from 5 to 35 mPaxc2x7s including a resin having a film-forming temperature of not higher than 0xc2x0 C. and a glass transition temperature of not higher than 0xc2x0 C., a particulate colored resin having a particle diameter of from 1 xcexcm to 20 xcexcm and water as disclosed in U.S. Pat. No. 5,621,021. In an attempt to prevent the trace of the writing from being left unerased when rubbed, the foregoing erasable ink includes colored resin particles having a great diameter which can less penetrate deep into paper. Nevertheless, the particulate resin used has a particle diameter which is not so small enough and thus can easily penetrate deep into paper. Thus, the particulate colored resin exhibits an enhanced adhesivity to the surface of paper and to each other. Accordingly, the foregoing erasable ink differs greatly in rubber-erasability from paper to paper.
Further, the use of the foregoing conventional rubber-erasable ink compositions for important document such as official document which allows no alteration and document containing date and amount of money is limited.
It is an object of the present invention to provide ink composition which can eliminate the foregoing disadvantages of the foregoing erasable ink compositions, that is, can not only be fairly erased by an eraser but also be fairly retained on paper.
As a result of the inventor""s extensive studies of such ink composition, the inventors paid their attention to the fact that the use of finely divided particles having a particle diameter of not greater than about 1 xcexcm as at least one of coloring element and bonding element has an adverse effect on the rubber-erasability of the conventional ink compositions.
The foregoing particles concerning coloring element and bonding element can penetrate less deep into paper or can easily prevent themselves from penetrating deep into paper if they have a sufficient particle diameter. Taking into account the normal distribution of the particles which have just been prepared, if the particles are distributed in a range of from 2 xcexcm to 20 xcexcm they can almost prevent themselves from penetrating deep into paper. These conditions must be applied both to coloring particles and bonding particles. In other words, even if the particle diameter of the coloring agent falls within the range of from 2 xcexcm to 20 xcexcm, when the particle diameter of the bonding particles falls within the range of from 0.1 xcexcm to 1 xcexcm as in general-purpose resin dispersion, the bonding particles penetrate deep into paper to strongly fix the coloring particles to the surface of paper from behind, making it impossible to obtain a good rubber-erasability. On the contrary, if the particulate coloring agent has a particle diameter of from 0.1 xcexcm to 1 xcexcm, the coloring agent itself penetrates deep into paper regardless of the particle diameter of the bonding particles, making it impossible to obtain a good rubber-erasability. It can therefore be presumed that all the conventional ink compositions are liable to penetration of particulate coloring agent and/or adhesive into paper and thus can attain neither good rubber-erasability nor high fretting resistance. It was thus found concerning the relationship between the penetration of particles into paper and the particle diameter that if the particles concerning coloring element and bonding element have a particle diameter of not less than about 2 xcexcm, the penetration of particles into paper can be lessened or eliminated. The present invention has thus been worked out.
Further, an unprecedented knowledge was found that the incorporation of xe2x80x9cheat-sensitive elementxe2x80x9d in a particulate colored ink composition containing coloring agent particles and bonding particles or the incorporation of xe2x80x9cheat-sensitive elementxe2x80x9d having particles distributed in a particle diameter range of from 2 xcexcm to 20 xcexcm in an ink composition containing particles having coloring properties and adhesive properties makes it possible to provide a writing that can be permanently fixed when subjected to heat treatment. The present invention has thus been worked out.